Where Roofs Fail

Posted by on Nov 29, 2016 in Building Surveying, Education

Where Roofs Fail

It’s obvious really: roofs fail at junctions and intersections.  That’s where all the stresses are and all the complexity.  Also, perhaps surprisingly, most roofing systems rely on lapped rather than sealed joints.  The expectation is that water will fall from the sky, or perhaps be blown by the wind, but the top of a building doesn’t have to be constructed like the bottom of a boat, not yet anyway.

The junctions between slatework and other materials are a particular case in point.  In Scotland slatework is generally nailed to timber boarding that entirely covers the roof structure.  In England, by contrast, slates are nailed to slender timbers called “battens” which in turn are nailed to the roof structure with gaps between them.  Roofers in either country are baffled by the technology in the other.  However, in both cases when slated roofs meet gable walls there is a need to provide some sort of weathering.  In Scotland there are two or three traditional methods for doing this.  One is to use mortar fillets to plug the gap.  These fillets are liable to crack and let water in and have come to be seen by many as inferior.  Another common detail is to insert a narrow gutter of metal which is terminated over a fillet of timber under the edge of the slates and is tucked into a groove cut into the wall.  This can be further improved by making it in two parts so that the piece tucked into the wall is separate from the piece that goes under the slates. This allows for movement that can cause the metal to crack.  Then there is a kind of metal flashing that extends over the slatework and is tucked into the wall.  It can quite easily be defeated by windblown rain.  In English construction there is less support for these arrangements and instead they more commonly use things called “soakers” interleaved between the slates and covered by a flashing tucked into the wall.

Series of images showing crowstepped roofs as examples

The disrepute into which mortar fillets have fallen is, arguably, unjustified.  They are quite a neat and economical solution with one proviso:  they must not be stuck to the slates and the wall at the same time.  This makes them crack quite quickly.  If, however, they are separated from the slatework with building paper, felt or even grease, the tendency to crack is much reduced.  Further refinements can be added to skew fillets, including soakers and the mortar can be reinforced with mechanically fixed expanded metal lathing, preferably of stainless steel.  It also helps to tilt the slatework up a little where is abuts a gable wall.

The narrow gutter arrangement which is sometimes called a skew gutter or a “watergate” is claimed to be superior.  However, it has some drawbacks.  It is expensive.  In a very heavy downpour skew gutters often overflow and leak into the roof.  They can be prone to blockage by twigs and leaves and then water is channelled under the slates and into the building.  They can be improved in situations where very large amounts of water are anticipated with a timber “roll” (sometimes known in the west of Scotland as a “bottle”) to make the main part of the gutter deeper.  Another option is to use the English method and to provide soakers and a cover flashing.  This actually works well with a traditional Scottish slated roof and provides no opportunity for water to overflow into the roof and does not have the problem of cracking that is the criticism of mortar fillets.

Quite often there is insufficient upstand at gable ends between the surface of the slatework and the top of the wall.  This can sometimes be especially problematical when the gable is “crowstepped”.  Where the wall is finished with a cope the only options are to provide a mortar fillet or soakers with a cover flashing taken right over the top of the wall or tucked into a groove cut into the top of the wall.  Mortar fillets are especially difficult to get to work in this situation.

A series of 4 images demonstrating roofs and their joints as examples

No part of a building is more important than its roof which is why, when we carry out building surveys we give roofs a lot of attention and when we undertake refurbishment projects we give a lot of thought to the design of roof coverings.